Development of the optic tecta in the frog Limnodynastes dorsalis

In Limnodynastes dorsalis neurogenesis of the optic tecta and the pattern of cellular lamination was determined by [3H]thymidine autoradiography. There was a rostral to caudal gradient of cell proliferation with peak neurogenesis in mid-larval life and by metamorphosis generation was complete. The cellular layers formed as each portion of tectum was generated. As evidenced by comparison of animals killed within 24 h of [3H]thymidine injection and those injected as larvae and killed at more mature stages, cells were generated in the ependymal and peri-ependymal layers and during development they migrated to form the peripheral layers. During late larval life, the soma diameter of cells in the most superficial layers, and the width of these layers increased. Tectal innervation during development was assessed by tracing axonal trajectories with horseradish peroxidase. Innervation recapitulated tectal growth with more caudal areas being progressively encompassed by primary optic fibres. By metamorphosis, the entire tectum was innervated. These results indicate that tectal generation, cellular lamination and innervation by optic fibres have a rostral to caudal gradient of development which is complete by metamorphic climax. Therefore to accommodate postmetamorphic retinal growth the developing retinotectal projection must remain labile beyond metamorphosis.